1. Field
This disclosure relates to wind turbines and wind collectors for horizontal axis wind turbines (HAWT) that are designed to harvest energy from the wind and transmit electrical energy to power storage or sent directly to end user for immediate energy consumption (grid-tied). More particularly, the present disclosure describes a compound rotor wind turbine that is enclosed within two separate annulus (annular wings) to provide for higher efficiencies in wind energy extraction based on coefficient of performance derived from the Lanchester-Betz-Joukowski limit.
2. Description of Related Art
Horizontal Axis Wind Turbines (HAWT) has been largely dominated by un-ducted/shrouded, mono-propeller turbine(s) as the primary configuration for wind energy extraction. The first documented patent for ducted/shrouded diffusers was the “Eolienne Bollee”, issued to Ernest Sylvain Bollee and Auguste Sylvain Bollee of France in 1868, #79985 and revised in 1885-#167726. The Eolienne Bollee consisted of fixed stator(s) preceding the rotor enclosed within a duct. Diffuser Augmented Wind Turbines (DAWTS) pioneered by Grumman Aerospace, a type of HAWT, U.S. Pat. No. 4,075,500, “Variable Stator Diffuser Augmented Wind Turbine Electrical Generation System” issued Feb. 21, 1978 to Oman & Foreman describes a diffuser/shrouded augmented wind turbine with variable stators preceding the rotors to control flow onto the rotors and yaw alignment of the turbine system. The diffuser shroud which encases the vanes and rotors is as such where the inlet shroud for vane & rotor placement is significantly less in diameter than the flanged diffuser exit. Such design claims to induce lower pressure along the back face, trailing edge area, downstream of the spinning rotors thereby improving rotor efficiency and rotor rpm as well as augmenting mass flow entering ahead of the rotor blades which facilitates more power extraction from the usable wind supply per rotor diameter.
A second DAWT type system for HAWT is proposed by U.S. Pat. No. 7,018,166, “Ducted Wind Turbine”, issued Mar. 28, 2006 to Gaskell featuring another diffuser shroud with two rotors, primary inlet rotor and second free rotor mounted downstream of the first rotor, outside of the diffuser shroud periphery. The blade tips of the free/second rotor are in the free-stream wind and a portion of the free or second rotor is exposed to the diffuser with supplemental force coming from the low pressure flow exiting the forward diffuser shroud. Both rotors are mechanically connected to a primary, generator. These types of diffuser/shrouded turbines have a primary purpose and that is concentrating or augmenting mass flow of the wind via the duct/shroud to improve rotor efficiency at the throat of a convergent/divergent diffuser for maximum velocities.
A problem common to diffuser type shrouds is flow separation from the shroud inner surface walls which negates the benefit of the diffuser shroud and causes pressure rise which may adversely affect rotor/propeller performance and thus harm efficiencies. Further, problems and complications of the diffuser design (DAWT) is the large exit to inlet (or nozzle to exit) diffuser ratios required, to form lower pressure gradients at exit. Additionally, low aspect ratio ducts, shrouds and diffuser in relation to the nozzle and exit area has led to problems with buffeting, yaw control due to air entrainment and high drag systems inherent in the design (higher pressure). Turbulent flow fields are also generated at the exit ducts of DAWTS, leading to efficiency and performance losses due to the increased diameter of irrotational and expanding wake flow.
Other concentrated wind systems or diffuser type systems for flow efficiency improvement in wind turbines, include prior art that attempt to maintain boundary air layer attachment at the diffuser walls by including orifices to re-direct free-stream airflow such as U.S. Pat. No. 4,075,500, previously cited, and U.S. Pat. No. 4,422,820, “Spoiler for Fluid Turbine Diffuser”, issued Dec. 27, 1983 to Kirsch & Markow. U.S. Pat. No. 5,836,738, “Advanced SuperVenturi Power Source”, Finney, describes three separate annulus merging flows to combine a singularity in flow stream to drive an actuator disk or turbine at the highest possible flow efficiency. All prior art of ducted (concentrator type)/diffuser augmented wind turbines (DAWTS) still do not address the problem of expanding irrotational wake flow that can disrupt flow and wind supply in a wind turbine array and lead to lower overall performance and degradation of efficiency of proximate wind turbines sited in wind farms because of turbulent flow. Furthermore, ducted/shrouded turbines of the prior art are prone to ice formation in freezing conditions due to the increase in wetted area of the duct. Ice-build up causes gross inefficiency and performance losses as the result of flow separation, blockage and change of surface geometry from ice accumulation.
The proposed art, Adaptive Control Ducted Compound Wind Turbine, takes a multidisciplinary approach to the design configuration, integrating ducted fan technology (aviation-aerodynamics), specifically the use of a positive cambered duct (airfoil geometry), vortex generators, internal duct heating to prevent ice formation, fixed and variable stator vane(s) and nacelle to augment, streamline and disrupt fluid flow in ducted fan/ducted wind turbine systems. The present embodiment and utility application is a continuation of U.S. Provisional Patent Application 61/219,347, “High Efficiency Ducted, Compound Rotor Turbine”, Sammy (also author of present embodiment), describes a novel configuration for a ducted wind turbine applying ducted fan technology.
3. Background Description of Physical Operation of Wind Turbines
The theory predicting the maximum amount of useful power that can be extracted from fluid flow was first written by English aerodynamicists, F. W. Lanchester in 1915. In 1920 Alfred Betz, a German aerodynamicist, and Nikolai Zhukovsky also spelled Joukowski, a Russian aerodynamicist, both published their results independently on the same body of work relating to maximum power extraction from fluid flow in an propeller or airscrew. History has long recognized the Betz Limit, so named for Alfred Betz's publication in 1920 as the governing law of power extraction from fluid flow in wind turbines. In fairness to past researchers, all came to the same conclusion independently that the useful amount of power that can be extracted from fluid flow within a stream-tube is 59.265% or as expressed as a coefficient of performance, C.sub.p=0.593.
The reference to the maximum limit will then be cited throughout this publication as the Lanchester-Betz-Joukowski limit. Further work by Froude and Glauert resulted in reference to the wind turbine as an actuator disk, propeller/airscrew, deriving an axial induction factor (losses) of 33.33%, 0.3333 or ⅓. The axial induction factor represents the amount of losses of kinetic energy in the wind that is converted to heat then dissipated through viscous shear as it passes the actuator disk within the stream tube. ⅔ of the kinetic energy in the wind is then available for useful power extraction.
Further loss of efficiencies lowering coefficient of performance in wind turbines un-ducted/ducted is due but not limited to propeller, generator, gearbox, and transmission inefficiencies, and poor yawing reaction control—a function of unsteady aerodynamics in wind turbines.
4. Background Description of Non-Planar Surfaces/Wings
Annular Wings are non-planar surfaces that may have a toroidal geometry in its most basic form. The benefits of non-planar wings have been documented theoretically and experimentally. Much research has shown that non-planar surfaces (annular wings, a.k.a. ring wings) significantly reduce wave drag due to high Oswald (span wise) efficiencies. Early study on non-planar wings conducted by Prandtl and Munk investigated the physics and determined theoretical calculations for non-planar wings in the form of bi-planes. Non-planarity for confined wingspan improves wing efficiency by capturing a larger volume of air to generate lift.